MFC r258078,258079

Increase the stack size for ppc64 from 4 pages to 8.

I found a stack overflow when a coredump was taken onto a ZFS volume with
heavy network activity. 2 DSI traps, plus one DECR trap, along with several
function calls in the stack, overflowed the 4 pages. 8 page stack fixes this.

Discussed with: nwhitehorn
Approved by: re
Relnotes: yes